In the manufacture of clutches and brakes, various techniques and fasteners have been utilized to assemble dry friction materials to support members. For example, welds, rivets, bolts and adhesives have been employed. In addition, ceramic filled compacts have been sintered, coined for density and size, and formed on a binder base by rivets, welds or with adhesives. Another assembly technique requires the forming of studs on the friction material as mating members for holes in the support member, and thereafter securing the friction material with adhesive to the supporting member.
A vehicle generally includes a clutch in the drive train, connected between the internal combustion engine and the drive wheel(s). Because an internal combustion engine has a relatively narrow speed range over which it is most efficient, a transmission with three or four different gear ratios is employed to maintain engine operation within the most effective speed range. Changes between these gear ratios are made with a clutch, preferably one able to transmit maximum torque without loss of power due to slippage. At the moment of clutch engagement, the impact must be absorbed and torque must be transmitted to the drive-train in a gradual and uniform manner, or the resulting shock could cause damage to the clutch or drive-train members.
The shock-absorption of a clutch at engagement has been accomplished with various lining constructions. There are certain advantages of a lining riveted to cushion springs such as good shock absorption, but at higher engine rpm ranges, that is about 7500 to 8000 rpm, a riveted lining disc may break due to centrifugal force, at the rivet heads. On the other hand, a lining material adhesively bonded to the clutch disc has no compressibility, but it provides an increased contact area and is much less likely to "chunk off" or burst at higher rpm ranges. It would be desirable to obtain the burst strength resistance characteristic of the bonded lining, and add the shock-absorbing characteristic of linings riveted to cushion springs. This can be done by bonding a lining to a metal disc for strength and riveting the metal disc to cushion springs, but this requires added manufacturing steps. In addition, elimination of the rivet as a joining technique has been sought, and adhesives have been employed for such joining.
One bonding method for assembling a friction material on a clutch disc provides an assembly with the two elements joined in a laminate structure, bonded under pressure with an adhesive material of unknown or presumed thickness. Generally when utilizing a bonding material the adhesive thickness is on the order of 0.005 inches or less and it is a continuous film with no appreciable compressibility in the adhesive layer. This bonding technique does not allow for the adhesive thickness to be uniform. Nor does it maximize the adhesive usage, as some adhesive is squeezed out during the compression of the laminated structure. The tensile strength of a cured adhesive often is dependent upon its thickness, and generally there is a preferred range of optimal thickness for a given adhesive. This optimal thickness may depend upon the texture of the surfaces being joined, that is, a rough, irregular surface will generally cause more variations in adhesive thickness than will a smooth, regularly spaced or textured surface. Thus, in the case of the two smooth surfaces, a quantity of the adhesive may be squeezed from between the surfaces, resulting in less than the required range of optimal thickness for maximum tensile strength. In the case of the irregular surface or surfaces, the separating distance between laminate surfaces will be suspect, as will the tensile strength of the adhesive bond.
The problem of attaining the optimal thickness range when utilizing adhesive has been attacked on several fronts. One effort involved the controlled spraying of a substrate with a latex adhesive of a nominally fixed viscosity to produce individual droplets as raised, spaced deposits. This required a control of the spray process, including the droplet velocity and the adhesive viscosity. An alternative to this involves the rolling of an adhesive, that is, a transfer roll method. However, this requires a specially textured roll. In the past the rolling technique has led to a uniform but continuous layer which does not resolve the problem of the material being squeezed from between two laminating surfaces. As a result of this squeezing process, there may be a lack of uniformity in the thickness of the bonding agent and a consequent loss of tensile strength. A technique utilizing incompressible beads of known diameter intermixed with the adhesive insured a fixed gap size between two hard, smooth surfaces. The hard incompressible material is not desirable in clutch applications where a certain flexibility is desired.
In the particular case of clutch plates and brake shoes, a technique of sintering preformed ceramic filled compact materials, coining these sintered preforms for sizing tolerances, and welding the coined product to backing plates has been utilized to produce clutches and brake shoes. This rather complex and time consuming procedure produces a friction material that can wear down to the bonding medium with no concern for rivets, and thereby attain the full utilization of the friction material before replacement. Although such a method attains nominal improvements in impact strength and shearing forces, it does so by increasing the processing steps and manufacturing time.
There have been other techniques developed to bond friction type material, including the use of heating or drying ovens to cure adhesive type materials. Any technique that bonds the clutch facing to the driven disc must provide the compression and shear strength to withstand clutch engagement.
An important object of the invention is to obviate the use of rivets, shock-absorbing springs, bonded metal reinforcing backing discs, and the pitched wafers for mounting friction material in a clutch assembly while retaining the desirable effects of these devices.
Another object of the invention is to maximize the use of adhesive material in attaining the desired physical parameters for any individually specified application.